Inductive power transfer (IPT) systems are used in a number of applications to transfer power wirelessly using an inductive or magnetic field coupling. In general, an IPT system comprises two main parts or “sides” which are loosely coupled: a primary side comprising a primary conductive path, commonly referred to as a “coil”, “track”, or “pad” depending upon the application of the IPT system, supplied with a high-frequency alternating current (AC) by a power supply; and a secondary side comprising one or more pick-up circuits inductively coupled with the primary conductor in use, and configured to supply power to a load. The high-frequency alternating current produces a changing magnetic field about the primary conductor, which induces a voltage at the secondary pick-up coil to power the load. The secondary coils in IPT systems are commonly movable with respect to the primary, resulting in a variable coupling due to misalignment or sub-optimal separation.
A particular example of an application of an IPT system is in the powering of implanted biomedical devices, termed transcutaneous energy transfer (TET) in the biomedical field. Previously, an implanted biomedical device such as a heart pump would have been powered from an external battery or other power source by a percutaneous cable, which is a major source of infection. TET systems, on the other hand, integrate or implant an IPT pick-up circuit along with the implanted biomedical device, and are thereby capable of transferring power across the intact skin, eliminating the percutaneous power cable and its associated risks. Other example applications of IPT systems include powering automated guided vehicles (AGVs), and wireless recharging of batteries in a wide range of devices from consumer electronics (such as mobile phones) to electric vehicles (EVs).
In many IPT systems, a power flow controller is desirable to ensure the load always receives the required power level under changing coupling level and load. This is clearly of particular importance in TET systems where the load may comprise a heart pump or artificial cardiac pacemaker, for example, where inadequate (or excess) power transfer may put the health of the implant recipient at risk.
To minimise the size, weight and/or heat generated by the pick-up circuit (which may be movable or implanted, for example) in an IPT system, it is sometimes preferable for the power flow to be controlled from the primary (i.e. external or non-implanted) side of the system.
Si et al. (P. Si, et al., “A Frequency Control Method for Regulating Wireless Power to Implantable Devices,” Biomedical Circuits and Systems, IEEE Transactions on, vol. 2, pp. 22-29, 2008) discloses a variable resonant frequency method to regulate the power flow by changing the effective capacitance of the primary through controlling the effective conduction period of additional capacitors parallel to the primary MOSFETs. Dissanayake et al. (T. D. Dissanayake, et al., “Experimental Study of a TET System for Implantable Biomedical Devices,” Biomedical Circuits and Systems, IEEE Transactions on, vol. 3, pp. 370-378, 2009) discloses an implementation of this frequency control in a TET system for powering heart pumps. However, this technique requires the power supply to have at least four switching devices and a plurality of switched capacitors.
International Patent Publication No. WO 2009/091267 similarly discloses an IPT system which varies the resonant frequency by switching parallel capacitors in and out of the circuit. These additional components may increase the size and cost of at least a part of the IPT system, and can potentially also affect reliability.
In other applications of IPT systems, however, it may be more important to minimise power losses by performing full wave zero voltage switching (ZVS) rather than hard switched regulation. Alternatively, or additionally, there may be applications in which it is necessary or desirable to easily switch between primary-side power flow control and ZVS modes of operation.
Zero voltage switching requires the power supply switches to be opened or closed (i.e. switched off or on, respectively) only at the moments when the instantaneous voltage of the AC waveform is substantially zero (i.e. crossing from a positive to negative voltage, or vice versa). This places less stress and reduces power losses in the switching transistor, and minimises electromagnetic interference (EMI).
However, existing circuits and methods for start-up and ZVS operation require fast feedback loops and initial DC currents at the primary DC inductors to ensure the existence of ZVS points, otherwise switching elements may be damaged.
The circuits of the prior art which provide for either primary-side power flow control or full-wave ZVS switching are generally substantially different. Combining those conventional circuits to achieve economies of scale and/or IPT systems which can easily fulfil either purpose is, therefore, unrealistic.
A further disadvantage of many IPT systems of the prior art is the common requirement for either uni- or bi-directional communication between the primary and secondary sides of the system. This is commonly overcome by including a radio frequency (RF) wireless communications link (typically comprising at least one additional integrated circuit and antenna on each side of the system) between the primary and secondary sides. This adds to the cost, size, complexity, and power requirements of the system, in particular where such a high-bandwidth communications channel is excessive for the communications requirements which may be as simple as merely transmitting on/off commands.